The field of art to which the invention relates comprises end fittings that retain and seal at least one open end of flexible plastic pipe for attachment to suitable utilization apparatus.
A flexible flowline is comprised typically of several distinct concentric layers that afford strength and flexibility to the flowline. The innermost layer, known as the carcass, prevents collapse of the flowline due to external hydrostatic pressure. The immediately adjacent layer, known as the barrier, is constructed from a synthetic polymer material and serves to prevent content leakage. There may also be succession of other layers of varying materials forming the overall composite of the flowline.
A flexible flowline may be utilized, for example, as a dynamic riser to couple a rigid flowline or another flexible flowline on the seabed to a floating vessel or buoy to convey production fluids such as oil, gas or oil/gas mixtures under pressure from an oil/gas well or platform to the vessel or buoy. An end fitting can be utilized to couple the flexible flowline at each end to an adjacent flowline or wellhead and the vessel or buoy.
One or more leak-tight seals contained in the end fitting has been achieved in the manner of the prior art utilizing a seal ring which, for example, encircles and engages the external surface of the barrier layer. This ring holds the barrier and carcass layers in position within the end fitting by a frictional force generated from the pressure of the seal ring on the barrier layer.
While functioning well, problems with the foregoing seals have arisen inside the end fitting where the flowline and end fitting are subjected to extreme fluctuations in temperature. Such extremes are known to occur in the flowline and end fitting when they are, for example, subjected to repeated changes in temperature as where, the flowline may be exposed to a temperature in excess 100xc2x0 C. when production fluid is conveyed through the flowline and subsequently may be exposed to temperatures on the order of 0xc2x0 C. from sea water when flow of the production fluid is interrupted or discontinued.
Repeated thermal cycling in the foregoing manner can result in a loss of seal integrity due to stress relaxation and creep of the barrier layer, followed by loss of compression at the seal. That is, with the seal area being highly stressed in compression, the flowline material at the beginning of service is often in its most highly plasticized state. This, however, deteriorates with time until ultimately a loss in barrier thickness occurs and a state of equilibrium is reached. Once compression at the interface of the seal ring and barrier declines to a low but non-zero level, loss of fluid content in the flowline can occur. Phenomena such as thermal expansion and creep can work to reduce or eliminate compression between the seal ring and the polymer layer.
Various forms of end fittings are disclosed in U.S. Pat. Nos. 1,817,776; 2,264,480; 2,341,164; 2,444,380; 3,995,897; 4,773,680; and 5,639,128.
Despite recognition of the foregoing, it has not been known heretofore how to satisfactorily eliminate the loss of tension and sealing of the flowline that can occur as described.
It is therefore an object of the invention to provide an enhanced seal for an end fitting capable of increasing the sealing properties against a flowline received therein.
It is further object of the invention to effect the previous object with a seal that is operative to compensate or be unaffected by adverse thermal effects encountered during service.
It is a further object of the invention to effect the previous objects with a seal of economical construction so as not to contribute to costly increases in the overall cost of the fitting.
This invention relates to an improved seal construction for an end fitting in which an open end of flexible flowline is to be received. More specifically, the invention relates to such an end fitting in which a novel seal construction is provided capable of functioning to compensate, offset or be unaffected by the adverse effects of temperature swings to which the fitting is exposed in service.
The foregoing is achieved in accordance with the invention by means of one or more seal units containing resilient seals of selected shape and assembled into a seal housing ring having a tapered outer surface received in a complementary recess of a surrounding wall. The housing ring may contain a sole or opposite resilient seals assembled thereon. The ring with seals is then axially forced into the surrounding recess by means of a displaceable drive ring. This reduces the inside diameter of the housing ring by forcing it onto a mating part of the end fitting which has a diameter smaller than that of the housing ring. As a result of the foregoing, the seal, or seals provide a means to seal against an irregular surface which may, for example, be an extruded polymer tube such as the flexbarrier of the received flowline. By forcing the housing ring onto the polymer tube, any surface irregularities are gradually reduced until any gaps between the housing ring and the polymer tube are eliminated. By eliminating the gaps, the problem of potential extrusion of the resilient seal is likewise eliminated.
A key to operability of the seal unit is the use of a compressible metal or relatively hard polymer housing for the seal to deform the mating layer in such a way as to eliminate the gaps. For these purposes, the seal housing may comprise an annealed corrosion resistant metal, a relatively hard polymer such as polyphenylene sulfide or other deformable material compatible with fluids to be conveyed through the flowline. Such construction can be used to seal high pressures since extrusion of the resilient seals is precluded. At the same time, the seal is superior to existing mechanical seals used in flexible pipes or hoses since mechanical seals require mechanical compression between the seal ring and the polymer layer. Moreover, the construction is not limited to the fluid sealing layer of flexible pipe since it can be readily applied to any extruded polymer layer of a flexible pipe or hose.